It is known that in a given passage, at points which are sufficiently far from the walls of the passage, the stream lines follow paths which are substantially parallel to the walls of the passage formed by the concave and convex surfaces of the blades. At all points along the path, the centrifugal force which is exerted on a particle is balanced by the pressure forces. The result of this is, generally, that the concave surface of the blade is subjected to a higher pressure than is the convex surface of the other blade which delimits the passage.
It is also known that in the boundary layer near the lower plate and upper plate, the speed of the fluid is low. It follows that the pressure forces are no longer balanced and the stream lines are curves perpendicular to the isobars and follow paths of considerable slippage in each passage from the concave surface to the convex surface as is well known to the person skilled in the art (see, for example, the article in the November 1941 French issue of the Brown Boveri review--p. 356 to 361 and, in particular FIGS. 2 and 3).
The slippage generates a counter-clockwise eddy against the upper plate of the passage and a clock-wise eddy against the lower plate as seen by an observer placed downstream from the set of blades.
These disturbances cause important losses known as secondary losses and the smaller the ratio between the height of the blades and the chord, the more the efficiency of a set of blades is reduced.
A known means for reducing the secondary losses consists in reducing the aerodynamic loading on the blades. This is equivalent to reducing the difference in average pressure between their concave and the convex surfaces e.g. by reducing the blade spacing to chord ratio of the set of blades.
However, the disadvantage of the above method is that it increases the friction losses in the zone of good flow along the passage in such a way that the gain obtained on the secondary losses can be cancelled by increasing the friction losses in the main flow.